Devices for capturing and storing bodily fluid intravaginally are commercially available and known in the literature. Intravaginal tampons are the most common example of such devices. Commercially available tampons are generally compressed cylindrical masses of absorbent fibers that may be over-wrapped with an absorbent or nonabsorbent cover layer. A means for withdrawing the tampon may include a withdrawal string, which may be attached by various means. It is important that the string be securely attached to the tampon with sufficient strength such that it is capable of withdrawing the tampon without the string breaking or disengaging.
The tampon is inserted into the human vagina and retained there for a time for the purpose of capturing and storing intravaginal bodily fluids, most commonly menstrual fluid. The tampon may be inserted manually or by use of an applicator. Withdrawal from the vagina is accomplished by pulling on end of the string with a force sufficient until the tampon slides. Since the vagina exerts pressure on the tampon, the force required to disengage the tampon may be significant. Because of this, it is helpful to knot the end of the withdrawal string, thereby giving the user something to grip onto.
Examples of apparatuses and methods for making knots for use as withdrawal strings in tampons can be found in U.S. Pat. No. 4,836,587; U.S. Pat. No. 6,585,300; GB 1236348; and GB 1398817. These apparatuses are complex and not designed for the newer high speeds desired in current manufacturing. The string may be subjected to additional forces, which may weaken the string. Additionally, the resultant knot is has a loose configuration and may unwind during packing and the shipping of the product. For these reasons, there remains a need for a new, robust method and apparatus for tightly forming a knot in the end of a string, particularly in the process of making sanitary protection articles such as tampons.
GB 1236348 purports to disclose a device for forming knots in yarn, thread or the like. The device has a body made of two parts that have passages corresponding to the looped form of the knot being made. The passages are connected to a vacuum for drawing one end of the yarn through the passages. The crossing points of the passages are located in a plane of separation. The crossing points are separated by resilient tongues that permit the yarn to be pulled through the passages to form a knot. The resilient tongues permit the yarn to “break through” to pull the knotted yarn ends out of the channel without destroying the loop.
Knots invariably weaken the string they are made in. When knotted string is strained to its breaking point, barring any flaws or damage in the string itself, the string almost always fails in or near the knot. The same bending, crushing, and chafing forces that produce the friction that holds a knot in place are also responsible for unevenly stressing the string fibers and ultimately lead to the reduction of strength. The exact mechanisms that cause the weakening and failure are complex, and these mechanisms are the subject of continued study.
The relative knot strength, also called knot efficiency, is the breaking strength of a knotted string expressed as a percentage of the breaking strength of the string without the knot. There are many difficulties in determining the overall numeric knot efficiency for a given knot. This is due to the many factors that can affect the results of a knot efficiency test: the type of fiber, the style of string, the size of string, whether it is wet or dry, how the knot is dressed before loading, how rapidly the knot is loaded, whether the knot is repeatedly loaded, and so on. With those limitations noted, and in order to give a sense of how much loss of strength knots cause, most knots in common usage have an efficiency between 40% and 80%.
The tension from a load causes the string to work back through the knot in the direction of the load. If this continues far enough the working end will pass into the knot and the knot will unravel and fail. This behavior in knots can be worsened when the knot is repeatedly strained and let slack, dragged over rough terrain, or subject to repeated impacts such as against a mast or flagpole. Even with secure knots some slippage may occur as the knot is first put under real tension. This can be dealt with by leaving plenty of string at the working end outside of the knot and by dressing the knot cleanly and tightening it as fully as possible before loading. In some cases the use of a stopper knot or, even better, a backup knot can prevent the working end from passing through the knot, but it is generally better to use a more secure knot if one is observed to slip. In life critical uses backup knots are often added to already secure knots in order to maximize safety.
What is needed is a device that reliably and efficiently forms tightened knots in the loose end of a string.